Glutamate is the principal excitatory neurotransmitter in the mammalian brain and is known to participate in higher order processes, such as development, learning, and memory. As an excitatory amino acid (EAA), glutamate is also involved in neuropathologic events, including cell death, that result from excessive stimulation of post-synaptic neurons (i.e., excitotoxic damage). Glutamate binds or interacts with one or more glutamate receptors (GluRs), which can be differentiated pharmacologically into different classes and subtypes. In the mammalian central nervous system (CNS) there are three main subtypes of ionotropic glutamate receptors (iGluRs), defined pharmacologically by the selective agonists N-methyl-D-aspartate (NMDA), kainite (KA), and xcex1-amino-3-hydroxyl-5-methly-4-isoxazolepropionic acid (AMPA).
The iGluRs are ligand-gated ion channels that, upon binding glutamate, open to allow the selective influx of certain monovalent and divalent cations, thereby depolarizing the cell membrane. In addition, certain iGluRs with relatively high calcium permeability can activate a variety of calcium-dependent intracellular processes.
During a period of anoxia (e.g., cardiopulmonary resuscitation), ischemic stroke, epileptic seizure, and other types of CNS injury, GluRs of the NMDA, KA, and AMPA subtypes are overactivated (Choi D W [1992] Nuerobiol 9:1261-96; Zipfel G J et al., [2000] J Neurotrauma 10:857-69; Fountain N B. [2000] Epilesia 41 Suppl 2:S23-30; Tanaka H et al. [2000] Brain Res 886(1-2):190-207; Pujol R et al. [1999] Ann NY Acad Sci 884:249-254). The net result of this effect is a massive increase in the concentration of intracellular calcium, which in turn triggers a deleterious cascade of events leading to neuronal death (Sapolsky R M [2001] J Neurochem 76(6):1601-1611). Functional overactivity of iGluRs has also been implicated in a variety of neurodegenerative diseases, such as lateral sclerosis, Alzheimer""s disease, Huntington""s chorea and AIDS dementia syndrome (Tanaka H et al. [2000] Brain Res 886(1-2):190-207).
In the search for safe and efficacious neuroprotective agents, iGluR antagonists remain thought of as promising therapeutic drugs (Gagliardi R J [2000] Arq Neuropsiquiatr 58(2B):583-588). A minimum of 800 neuroprotective trials using GluR antagonists is currently underway worldwide. Among the neuroprotective agents being studied, the most important ones include iGluR antagonists of the NMDA and AMPA subtype, and inhibitors of glutamate release. However, many of these drugs cause significant side effects (e.g., neurotoxicity) that will probably limit their widespread clinical use. For example, the NMDA channel blocker dizocilpine (MK-801) causes neuronal vacuolation in specific areas of the rat brain cortex (Olney J W et al. [1989] Science 244:1360-1362; Fix A S et al. [1994] Drug Development Research 32:147-152; Muir K W et al. [1995] Stroke 26:503-513).
In contrast to the iGluRs, the metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors capable of activating a variety of intracellular second messenger systems following the binding of glutamate. Activation of mGluRs in mammalian neurons can decrease the activity of ion channels, including ligand-gated channels such as iGluRs. Several subtypes of mGluRs have been isolated by molecular cloning. In addition, the various subtypes of mGluRs have been divided into three groups based on amino acid sequence homologies, the second messenger systems they utilize, and pharmacological characteristics (Nakanishi [1994] Neuron 13:1031).
U.S. Pat. No. 6,084,084 (the ""084 patent) discloses a human mGluR protein and methods of screening for compounds that bind to the mGluR receptor and modulate its activity, and using such compounds to treat various neurological disorders. The ""084 patent also cites several references that teach various mGluR modulators, the majority of which are L-glutamate derivatives.
A family of substances that have not previously been investigated as modulators of GluR activity are the naturally occurring aromatic amino acids, L-tyrosine, L-tryptophan, and L-phenylalanine. While some phenylalanine derivatives have been investigated as possible agonists or antagonists of mGluR activity, these derivatives showed little or no agonist or antagonist activity on any subtypes of the receptors investigated (Sekiyama N et al. [1996] Br J Pharmacol 117:1493-1503).
Aromatic amino acids have been available as over-the-counter dietary supplements for some time, with their consumption generally viewed as beneficial for their role as biosynthetic precursors for the neurotransmitter precursors serotonin, dopamine, and norepinephrine. In addition, there is research suggesting that oral administration of neurotransmitter precursors may be useful in treatment of some pathologic conditions of the brain, but not others. For example, oral administration of preparations that included tyrosine was found to confer some benefit to patients suffering from multi-infarct dementia and Alzheimer""s disease (Meyer J S et al. [1977] J Am Geriatr Soc (July) 25(7):289-298), and oral administration of D-phenylalanine conferred some benefit on patients suffering from Parkinson""s disease (Heller B et al. [1976] Arzheim-Forsch (Drug Res.) 26(4):577-579); however, orally administered tyrosine did not confer a benefit to patients suffering from cocaine dependence (Galloway G P et al. [1996] J Psychoactive Drugs (July-September) 28(3):305-309).
It is evident that the currently available glutamate receptor modulators may be of limited use, both as research tools and potential therapeutic agents, as a result of their lack of potency and selectivity. Accordingly, there remains a need for safe and efficacious neuroprotective agents.
The subject invention concerns methods for treating a disease or condition which is related to, or which can be affected by, modulation of glutamate receptor (GluR) activity. Particularly, the subject invention concerns methods for treating neurological conditions characterized by excessive activation of glutamatergic ionotropic receptors (iGluR). The treatment can be either prophylactic in nature or to alleviate symptoms of such neurological conditions.
According to the methods of the subject invention, an AAA is administered in an amount effective to increase the concentration of the AAA within the brain to a level above physiologically normal. For example, the AAA can be administered in an amount effective to bring the patient""s AAA blood plasma level within the range of about 200 xcexcM to about 2000 xcexcM. Preferably, the patient""s AAA blood plasma level is brought to within the range of about 300 xcexcM to about 1800 xcexcM. More preferably, the patient""s AAA plasma level is brought to within the range of about 800 xcexcM to about 1500 xcexcM. However, the appropriate concentration of AAAs in the blood can be adjusted, as permeability of the blood-brain barrier can vary with different disease states.
Neurological conditions characterized by excessive activation of glutamatergic ionotropic receptors (iGluRs) include, but are not limited to, anoxic/hypoxic damage (e.g., cardiopulmonary resuscitation, drowning), traumatic brain injury, spinal cord injury, local anesthetic-induced seizure activity, ischemic stroke, ischemic neurodegeneration of the retina, epilepticus, Tourette""s syndrome, obsessive-compulsive disorder, drug-induced (e.g., nerve gas-induced) CNS injury, chronic pain syndromes, acute and chronic neurodegenerative disorders (e.g., lateral sclerosis, Alzheimer""s disease, Huntington""s chorea), AIDS dementia syndrome, cocaine addiction, or combinations thereof. In one embodiment, the method of the subject invention comprises parenterally administering to the patient an AAA, an analog or isomer of an AAA, or combinations thereof.
Unless otherwise indicated, as used herein, the term xe2x80x9cAAAxe2x80x9d includes naturally occurring aromatic amino acids (e.g., L-tyrosine, L-phenylalanine, and L-tryptophan), their isomers, including optical isomers (e.g., dextrorotatory (D-), levorotatory (L-), or mixtures (DL-) thereof), and analogs thereof. Mixtures of naturally occurring aromatic amino acids, isomers, and analogs, are also contemplated.
The present invention also concerns methods for modulating GluR activity. In one aspect, the method of the subject invention comprises lowering Glu concentration in the synaptic cleft in a patient by administering an AAA. The present invention further pertains to methods for attenuating GluR-mediated miniature excitatory postsynaptic currents (mEPSCs), either during normoxia or ischemic conditions, within a patient by administering an AAA. In addition, the present invention concerns methods for inhibiting GluR activity in a patient by lowering the concentration of Glu in a patient through the administration of an AAA.
The subject invention also concerns articles of manufacture useful in treating a neurological condition characterized by overactivation of an ionotropic glutamatergic receptor.